Radiation therapy is one of two primary treatments for clinically-localized prostate cancer (PCa) and is the principal therapy for locally-advanced disease associated with a higher grade, stage and/or PSA. While the success rate for both radiation and surgery is high for low-grade organ-confined disease, the estimated ten year disease-free-survival for advanced disease is less than 50%. Therefore, a means to improve the therapeutic index for patients with clinically-localized high stage and/or grade prostate cancer would significantly decrease the morbidity and mortality of this disease. In this proposal, a model is described to test the hypothesis that the therapeutic index for local treatment of prostate cancer can be improved by selectively sensitizing prostatic cancer cells to ionizing radiation. Here, the natural pathways of genomic DNA damage repair will be the therapeutic target. Inherited mutations in these pathways, such as in Ataxia Telangiectasia, result in cellular hypersensitivity to ionizing radiation (IR). We have previously shown that mammalian cancer cells be made similarly hypersensitive to IR by knocking down DNA repair protein levels through RNA interference (RNAi) therapy. While promising, this strategy requires a means to selectively target prostatic cells. We now propose three specific aims to develop a model for selective radiation sensitization of prostate cancer cells through targeted RNAi therapy. In Aim 1, we will determine the identity of additional novel gene targets in DNA repair pathways for enhanced radiation sensitization. Secondly, in Aim 2, we will develop an in vivo model for RNAi targeting and radiation sensitization through the use of a Prostate Specific Membrane Antigen (PSMA) targeting RNA aptamer developed in our laboratory. This aptamer, xPSM-AlO, has been previously applied to deliver RNAi therapeutics to prostate tumor cells in vivo. Lastly, in Aim 3, we will extend these pre-clinical studies to a phase I clinical trial to determine safety and selective target gene knock-down. These studies have great potential in developing the first tissue-selective radiation sensitization agent and in translating a novel strategy to decrease the morbidity and mortality of locally advanced prostate cancer.